One vacuum brake booster of this general type is, for example, known from DE-OS 42 27 479 A1. The valve body of this known brake booster, which is prestressed in the direction of the sealing seats by means of a valve spring, is provided with passages within the region of its sealing surface. These passages originate at an annular chamber that is connected to the work chamber and end in the pneumatic chamber, namely on the side of the valve body that faces away from the sealing seats. Consequently, a continuous pressure compensation takes place between this chamber and the work chamber. In this case, the valve body has two pneumatically effective surfaces. The first effective surface or annular surface is limited by the radially inner sealing seat (atmospheric sealing seat) of the control valve as well a radially inner sealing lip of the valve body which cooperates with the guide part that limits the pneumatic chamber. Consequently, this effective surface or annular surface is subjected to a pneumatic differential pressure between the atmospheric pressure and the vacuum that exists in the pneumatic chamber in the release position. This results in a force component that is directed opposite to the force generated by the valve spring and decreases during the actuation of the brake booster as the degree of ventilation of the pneumatic chamber increases, namely until said force component is reduced to zero once the control point of the brake booster is reached or the work chamber is entirely ventilated, i.e., the first annular surface is pressure-compensated.
The second effective surface or annular surface is limited by the radially outer sealing seat (vacuum sealing seat) of the control valve as well as a radially outer sealing lip of the valve body which cooperates with the inner wall of the valve gear casing which limits the pneumatic chamber, i.e., this effective surface or annular surface is pressure-compensated in the release position of the brake booster and subjected to a pneumatic differential pressure when the brake booster is actuated. Consequently, a vacuum-dependent force component that boosts the effect of the aforementioned valve spring is generated.
However, one disadvantage associated with previously disclosed vacuum brake boosters is that constructive modifications in the valve gear casing are required for minimizing the vacuum-dependent sealing force component that occurs when higher actuation forces are applied.
The present invention make it possible to minimize the aforementioned sealing force component without requiring constructive modifications of the valve gear casing.
According to the invention, this objective is attained by forming the radially outer limitation of the pneumatic chamber by the guide part.
According to one additional advantageous development of the invention in which the valve body comprises a radially outer sealing lip as well as a radially inner sealing lip, and in which the radially inner sealing lip cooperates with an inner, tubular region of the guide part, it is proposed that the radially outer sealing lip cooperates with a radially outer region of the guide part, which is realized as a cylinder and radially adjoins the valve gear casing.
One additional advantageous embodiment of the invention in which the valve body comprises a first annular surface that is limited by the radially inner sealing lip and the radially inner sealing seat as well as a second annular surface that is limited by the radially outer sealing lip and the radially outer sealing seat is characterized by the fact that both annular surfaces have the same size. Due to this measure, the vacuum brake booster according to the invention behaves in a pressure-compensated fashion, in particular, in its moderate operating range, namely because the forces that act in opposite directions are neutralized.
Another advantageous embodiment of the object of the invention is characterized by the fact that the first annular surface is larger than the second annular surface. This embodiment is particularly advantageous in the realization of control valves that can be controlled proportionally and, for example, are used in brake boosters that are actuated independently or electromechanically.
In this case, it is advantageous if the guide part is made of plastic or metal.
A simplification in the assembly of the control components in which the guide part is sealed relative to the valve gear casing by means of a ring seal can be attained with one additional embodiment in which the ring seal is arranged in a radial groove in the guide part.
A reduction of the axial length, in particular, the axial length of the brake booster control components in which an air filter, as well as a readjusting spring that prestresses the radially inner sealing seat (atmospheric sealing seat) opposite the actuating direction, is arranged within the air intake region of the valve gear casing, can be attained if the air filter axially adjoins the guide part and the readjusting spring is realized as a cylinder and arranged radially outside as well as coaxial to the air filter.